In a fluorescent display device, in order to obtain a variety of luminous colors, a phosphor emitting yellow to red light among various phosphors is important in increasing the variation of the fluorescent display device.
A ZnCdS:Ag phosphor has been used as a yellow to red light emitting phosphor. However, the present environmental restriction requires a reduction of Cd level, which is an environmental load material and a component of the phosphor. Thus, there are strong and concerted efforts in developing Cd-free phosphors having a high reliability.
There has been known a technique for using, as one of the Cd-free phosphors, an Ln2O2S:Eu phosphor (Ln is at least one selected from a group consisting of La, Gd, Lu and Y) phosphor (hereinafter, referred to as “Ln2O2S:Eu phosphor”) in the fluorescent display device.
For example, Japanese Patent Laid-open Publication No. H10-12165 discloses a technique wherein the amount of oxide on the surface of Ln2O2S:Eu phosphor is reduced below a specific value appropriately for the use of Ln2O2S:Eu phosphor, and the fluorescent display device is manufactured under a non-oxidative atmosphere by using an autolysis binder. As a result, the oxidation on the surface of the Ln2O2S:Eu phosphor is suppressed, thereby improving the initial brightness and reliability of the fluorescent display device.
Further, e.g., Japanese Patent Laid-open Publication No. H7-48570 discloses therein a technique for improving the reliability of a fluorescent display device using an Ln2O2S:Eu phosphor by forming a transparent protective film selected from Al2O3, SiO3, TiO2 and CeO3 on the surface of Ln2O2S:Eu phosphor.
In addition, e.g., Japanese Patent Laid-open Publication No. 2003-147354 discloses therein a technique for improving the reliability of a fluorescent display device using an Ln2O2S:Eu phosphor by adding a metal selected from Mg, Sr, Ba, Be and Ca onto the surface of Ln2O2S:Eu phosphor to reform its surface.
The Ln2O2S:Eu phosphor is a Cd-free phosphor capable of being used as a phosphorescent layer composed of a phosphor emitting a yellow to red light in response to electrons emitted from an electron-emitting source provided inside a vacuum-sealed vessel while withstanding actuating conditions of the multi-color fluorescent display device.
However, in the multi-color fluorescent display device wherein the Ln2O2S:Eu phosphor is used together with a phosphorescent layer containing at least one low-resistant oxide phosphor of a ZnO:Zn phosphor, a ZnGa2O4 phosphor and a ZnGa2O4:Mn phosphor, the brightness of the phosphorescent layer composed of the Ln2O2S:Eu phosphor deteriorates in a short time period.